The invention relates to the making of filaments and particularly to filament production wherein an elongated rod has a pendant drop formed thereon which is engaged by a rotating heat extracting member such as a copper disc.
Even more particularly, the invention relates to the above subject matter wherein filaments are produced from rods of high melting point material, e.g. those having melting points exceeding 1300.degree. Celsius.
With regard to the present invention, the term filament is meant to define slender, elongated metallic elements each having a transverse dimension less than the element's length dimension. Examples of such element's may include sheet, ribbon, or wire. One particular use for these products is as the combustible fill material within a photoflash lamp such as described in U.S. Pat. Nos. 3,535,063 (L. F. Anderson et al), 3,897,196 (J. P. Saunders et al), and 4,008,040 (D. E. Murray et al), all of which are assigned to the assignee of the present invention. The combustible fill material readily ignites upon activation to provide the intense flash typically associated with photoflash applications. At least two methods are presently used to produce elongated filaments from rods of metallic material. The first involves forming a suspended or pendant drop on the end of a vertically oriented rod and engaging the drop from below with a rotating copper disc. Such a process is described in U.S. Pat. No. 3,896,203 (Maringer et al). The disc has a chill surface which has a coefficient of thermal conductivity sufficient to withdraw heat from the molten drop in such rapid fashion so as to form the filamentary element on the surface of the disc. The filament is thereafter emitted and collected for future use.
A second method of producing metallic filaments from rods involves formation of what is termed a sessile drop. By sessile is meant a drop which is substantially upwardly projected using suitable means to engage a rotating casting disc located above the drop. In other words, the rod and disc are inverted in comparison to the arrangement described in the aforementioned suspended drop process. With regard to the present invention, the term pendant drop will be used and is meant to include both suspended and sessile drops.
Because of the requirement for either a vacuum or protective gas atmosphere when producing high melting point material filaments, a preferred means for melting the rod is induction heating. Unfortunately, the power requirements for providing the energy necessary to melt such materials are so high and vibrations arising from magnetic stirring and related electrical effects are usually established in the molten drop. These vibrations in turn result in the production of dimensionally inconsistent filaments due to variations in the dwell time of the filaments on the casting disc. The dwell time is defined as the period during which the formed filament resides on (adheres to) the disc's chill surface.
From the foregoing background, it can be readily understood that an apparatus and method which are capable of producing dimensionally consistent filaments from elongated rods of high melting point material by eliminating vibrations in the molten drop from which the filaments are emitted would constitute a significant advancement in the art.